Epidemiology & Geographic Distribution

In the United States, an average of 68 La Crosse encephalitis virus (LACV) neuroinvasive disease cases are reported each year. There is significant under-diagnosis and under-reporting of less severe cases of LACV disease, therefore reported LACV neuroinvasive disease cases are used to compare trends over time and place. To ensure standardization of reporting across the country, CDC recommends that the national surveillance case definition (standardized reporting criteria) be consistently applied by all state health departments.

LACV disease cases occur primarily from late spring through early fall, but in subtropical endemic areas (e.g., the Gulf states), rare cases can occur in winter. Historically, most cases of LACV neuroinvasive disease were reported from the upper Midwestern states (Ohio, Wisconsin, Minnesota, Indiana, Iowa, and Illinois). Recently, more cases have been reported from mid-Atlantic and southeastern states (North Carolina, Tennessee, West Virginia, Georgia, Virginia, and Kentucky).

Data Table: In the United States, the number of La Crosse virus neuroinvasive disease cases reported each year varies. From 2009 through 2018, an average of 68 cases were reported annually (range 31–116).

Data table: This map shows the distribution of La Crosse virus neuroinvasive disease average annual incidence by county of residence from 2009 through 2018. Counties are shaded according to incidences ranging from less than 1.00, 1.00 to 2.49, and greater than 2.50 per 100,000 population. Shaded counties are primarily distributed in eastern half of the country. Most of the highest incidence counties are along the Appalachian range.